A view of the nucleus of Comet Halley from the ESA Giotto spacecraft. (credit: ESA, MPAe, Lindau)

A chance of a lifetime: the missions to Comet Halley

by Andrew LePageMonday, March 14, 2011

Comet Halley, officially designated 1P/Halley by astronomers, is undoubtedly the most famous of all the periodic comets in our solar system. First observed in ancient times perhaps as early as 466 BC, its 76-year periodicity was first recognized by English astronomer Edmond Halley in 1695. The observation of its subsequent return to the inner solar system in 1759 was considered a triumph of the predictive power of the Newtonian laws of gravitation. With its return to the inner solar system in early 1986 (its first appearance since the beginning of the Space Age), Comet Halley was a tempting target for spacecraft exploration by the spacefaring nations of the Earth.

American mission proposals

The first serious studies into the requirements for a mission to Comet Halley were begun in the late 1960s. It was quickly recognized that there would be difficulties reaching this famous target. Its 76-year orbit is highly eccentric, coming as close as 0.59 AU (an AU or astronomical unit being the average distance of the Earth from the Sun) at perihelion and swinging out as far as 35.1 AU at aphelion. Unlike all of the current planets in our solar system, the orbit of Comet Halley is steeply inclined to the plane of Earth’s orbit known as the ecliptic. At an inclination of 162 degrees, this comet actually travels backwards relative to the planets. Launching a spacecraft into a highly eccentric, retrograde orbit to match that of Comet Halley would require enormous amounts of energy, which are far beyond what is practical with conventional chemical propulsion.

Some sort of low-speed rendezvous was considered to be the only viable option given what the assumed state of technology and interplanetary navigation would be in the 1980s.

One solution to this problem was to intercept Comet Halley as it passed through the ecliptic plane where the launch energy requirements would be more modest. As it approached perihelion, Comet Halley reached its ascending node (where the comet passed up through the ecliptic plane) on November 8, 1985 at a distance of 1.8 AU from the Sun - beyond the orbit of Mars in the asteroid belt. Low energy launch windows for a simple ballistic path from Earth to this point occurred in February and July of 1985. After passing perihelion on February 9, 1986, Comet Halley reached its descending node (where it passed back down through the ecliptic plane) on March 10 at a distance of 0.85 AU from the Sun. Low energy launch windows to this point occurred in July and August of 1985.

While the energy requirements for a descending node encounter with Comet Halley were slightly less owing to the fact that the spacecraft would be placed into a heliocentric orbit not too unlike Earth’s, the relative velocity of encounters at either point would be in excess of 60 kilometers per second (37 miles per second). Even though there were higher energy trajectories out of the ecliptic plane that could decrease the encounter speeds somewhat, it was the general opinion of the American science community that first-rate science could not be performed at such high velocities. Some sort of low-speed rendezvous was considered to be the only viable option given what the assumed state of technology and interplanetary navigation would be in the 1980s.

Undeterred, American engineers and scientists began to look into alternatives to the simple ballistic encounters. One family of trajectories examined involved using the powerful gravitational field of Jupiter. In one proposal, a Saturn V topped with a Centaur as a fourth stage could launch a probe in 1977 or 1978 for a fast, one-year trip to Jupiter where the spacecraft would be flung high out of the ecliptic plane into a retrograde orbit. The spacecraft would then encounter Comet Halley five to eight months before perihelion at a relative velocity of several kilometers per second. Unfortunately the Saturn V was very expensive and its availability uncertain (in fact, the Saturn V would be retired in 1973). The trip time of over seven years was also considered to be far too long given the state of the technology at the time. It was for this same reason that NASA’s decade-long Grand Tour proposal was downsized to the much shorter, four-year mission of Voyager to Jupiter and Saturn (see “The Grand Tour: Uranus”, The Space Review, January 24, 2011).

Another option involved using a probe fitted with ion thrusters. Much more efficient than conventional chemical-based systems, ion propulsion could provide the needed velocity change to rendezvous with Comet Halley. One proposal examined called for the spacecraft to be launched into an elongated solar orbit in 1978. Solar-powered ion thrusters would then gradually slow the speed of the receding spacecraft and, after almost four years, reverse its direction of travel around the Sun while it was far beyond the orbit of Jupiter. As the spacecraft fell back towards the inner solar system, the ion thrusters would continue to alter the orbit resulting in a low-speed rendezvous with Comet Halley a couple of months before perihelion. But once again the seven-year trip time was considered far too long. Even substituting a nuclear reactor as the power source (which entailed its own development issues) would cut only three years from the flight time. An even more efficient method of propulsion was required to cut down the flight time.

In the mid-1970s, the Jet Propulsion Laboratory (JPL) became aware of studies performed under a NASA contract by an engineer at the Battelle Memorial Institute named Jerome Wright. His work showed it was possible to employ a solar sail, which uses the pressure from sunlight for propulsion, to perform a low speed rendezvous with Comet Halley. By 1976 JPL started an in depth study of the proposal. The initial design called for a solar sail about 800 meters (2,600 feet) on a side supported by four diagonal cross beams carrying a payload of 800 kilograms (1,800 pounds). Launched by the Space Shuttle around 1981 and potentially assembled with the assistance of astronauts in Earth orbit, the solar sail would spend its first 250 days of flight slowly spiraling closer to the Sun. Using the enhanced light pressure from its closer 60-day solar orbit, the light sail would be used to pump up the inclination of the probe’s orbit over the course of the next nine months until it matched that of Halley. After further maneuvers to shape the probe’s orbit, a low speed rendezvous with the comet could then take place in early 1986.

By 1980 it was clear that the United States—the undisputed leader in planetary exploration for almost two decades—would not launch a dedicated mission to Comet Halley.

Because of the unknowns associated with deploying such a large structure in space and the questionable availability of the Space Shuttle, in early 1977 the square solar sail design was replaced by the heliogyro, which was invented by Richard MacNeal and John Hedgepath a decade earlier. Consisting of a set of 12-kilometer-long (7.5-mile-long) rectangular sails arranged like the blades of a helicopter, the blades of this solar sail concept did not require a rigid structure but used the centrifugal force from slowly spinning to maintain rigidity. And since the individual blades would be deployed using the centrifugal force to unreel them from a set of storage drums, the heliogyro concept would be easier to set up in space with no assistance from an astronaut. But even this innovative concept had too many unknowns and it was doubtful it could be developed in time for a launch in 1981. In September 1977 NASA officially abandoned the solar sail concept in favor of solar-electric ion propulsion.

It quickly became clear that the estimated $500 million price tag for this project (equivalent to about $1.7 billion today) was simply too large given the cost overruns of NASA’s increasingly delayed Space Shuttle program. By early 1978 NASA began to examine other options. One way to meet the science objectives of a Halley mission was to rendezvous with a short-period comet that would be easier to reach. One proposal that gained favor was a rendezvous with Comet 10P/Tempel 2 (not to be confused with the periodic comet 9P/Tempel visited by Deep Impact in 2005 and the NExT mission last month) using solar-electric propulsion.

The proposed 2,700-kilogram (5,950-pound) spacecraft would be launched by the Space Shuttle in late July 1985 and sent into interplanetary space using the all-solid IUS upper stage. Once on its way, the probe would deploy its huge solar arrays and start thrusting using a set of six mercury-fueled ion engines. Around November of 1985 the probe would make a distant encounter with Comet Halley when it was 1.53 AU from the Sun. Ten days before closest approach, a small probe supplied by ESA (European Space Agency) would be deployed to make a closer inspection of Comet Halley. With a mass of 150 to 250 kilograms (330 to 550 pounds), the spin-stabilized European probe would have been based on the successful ISEE 2 spacecraft design and would pass only 1,500 kilometers (930 miles) from Halley’s nucleus while the American mothership flew by at a safer distance of 130,000 kilometers (81,000 miles). The American ship would then continue its flight and rendezvous with Comet Temple-2 in July of 1988.

In the end, even this mission was never funded because of its high costs and the need to fund other planetary missions with higher priority like Galileo. While less expensive options for a fast flyby of Comet Halley and, later, other comets would be studied in the years and decades to follow, by 1980 it was clear that the United States—the undisputed leader in planetary exploration for almost two decades—would not launch a dedicated mission to Comet Halley. The consensus of the American scientific community was that a more affordable fast flyby was unacceptable and scientifically inadequate. While it was not realized at the time, the infamous hiatus in American planetary missions had already started because of overly ambitious goals at a time of shrinking budgets. Fortunately, other groups of scientists around the world did not share America’s somewhat myopic view.

International missions

After having their piggyback probe to Halley stranded after NASA’s cancellation of the Tempel-2 rendezvous mission, ESA decided to go it alone and launch their first deep space mission to Comet Halley. The Giotto mission, named after the Italian Renaissance artist Giotto di Bondone who included Halley as the star of Bethlehem in his 1304 painting “Adoration of the Magi”, was officially approved by ESA’s science committee on July 8, 1980, despite the criticism of the French. The spin-stabilized design of Giotto was based on the successful GEOS research satellites built by British Aerospace first launched in 1977. Because Giotto would penetrate deep into the dusty coma of Comet Halley, a major modification included the addition of a two-layer Whipple shield to the base of the probe to help protect it from cometary dust particles as large as one gram (0.03 ounces) during its 68 kilometer-per-second (42 mile-per-second) flyby. With a mass of 573.7 kilograms (1,265 pounds) at the time of its encounter, Giotto carried ten instruments with a total mass of 60 kilograms (130 pounds) to study Comet Halley and its environment including a multicolor camera to provide high-resolution images of the comet nucleus. Tracking of the fast moving nucleus was accomplished using specially designed control software to keep the comet in the camera’s field of view. All data would be transmitted live because it was not expected that Giotto would survive its pass 500 to 1,000 kilometers (300 to 600 miles) from Halley’s nucleus.

Originally Giotto was to be launched into a geosynchronous transfer orbit by an Ariane 3 rocket with another commercial payload riding along, and use a MAGE 1S solid rocket motor built into the spacecraft to send it into deep space. However, it proved to be impossible to find a commercial payload that would be available during the limited July 1985 launch window. Eventually Giotto was shifted to an Ariane 1 rocket as its sole payload. Even though the Ariane 1 now had ample power to launch Giotto directly into the required solar orbit without the MAGE 1S kick motor, the original launch profile and solid motor were retained due to the advanced state of development and the time needed to make changes.

All data from Giotto would be transmitted live because it was not expected that Giotto would survive its pass 500 to 1,000 kilometers from Halley’s nucleus.

The Giotto mission would not be the only one to Comet Halley. During the 1970s the Soviet Union had mounted a very successful series of missions to the planet Venus and were studying a joint mission with France designated Venera 84. Originally meant to deliver a pair of French-supplied balloons into the Venusian atmosphere, Soviet scientists were hoping to observe Comet Halley using their Venera orbiters just as the Americans planned to do with their Pioneer Venus 2 orbiter, which arrived at Venus in late 1978. As luck would have it, Comet Halley would pass only 40 million kilometers (25 million miles) from Venus and provide a better vantage point than Earth for observation.

During the early 1980s an even more daring mission was developed. Instead of the Venera orbiters observing Comet Halley from afar, it was found that it was possible for them to flyby Venus after dropping off their payloads and continue on to encounter Comet Halley in early March 1986. As a result of the change in mission, the French balloon payload had to be downsized and the Veneras would no longer be placed into orbit to support them. In the end the French decided to walk away from the balloon program, leaving the Soviets to build their own balloon payload instead.

In April 1982 the Soviet Union publically announced their plans to send a pair of Venera spacecraft, designated 5VK, to Comet Halley which they named VEGA (the Russian acronym for “Venera-Halley” where the “H”, which does not exist in the Cyrillic alphabet, is usually transliterated as a “G”). With a mission length of about 15 months, it would be the longest interplanetary mission attempted by the Soviet Union. In addition to the Venera lander and smaller balloon payload, the pair of VEGAs would carry 240 kilograms (530 pounds) of instruments including those supplied by 13 other countries from around the world. The 5VK spacecraft, with a mass of 4,920 kilograms (10,840 pounds), were modified versions of the very successful second-generation Venera built by NPO Lavochkin. In addition to shielding to protect the spacecraft from dust impacts, the 5VK also sported larger solar panels and carried an increased load of propellants as well as attitude control gas. Another major addition to the 5VK was a Czech-built pointable scan platform that carried, along with other optical instruments, a CCD-based television system jointly developed by the Soviet Union, Hungary and France. A tracking system would allow the platform to stay pointed at the comet despite the high encounter velocity and the uncertain position of the nucleus. There would be an unprecedented level of international cooperation on this mission. Given the uncertainty of the position of Halley’s nucleus, the VEGA spacecraft would serve as pathfinders for Giotto which was suppose to fly much closer to the nucleus.

While the Soviet VEGA spacecraft were the largest sent to Comet Halley and the European Giotto mission would pass the closest, very early on Japan decided that they also could attempt a much more modest but still scientifically useful mission to Comet Halley as well. During the 1970s Japanese scientists and engineers at ISAS (Institute of Space and Aeronautical Science—one of the precursors of the Japanese space agency JAXA established in 2003) began studies for a Halley probe launched using their Mu-3S all-solid, three-stage launch vehicle that was capable of orbiting a 300-kilogram (660-pound) payload. In 1979 the Japanese Halley mission was approved with six years to complete the project.

Early on it was decided to launch two spacecraft: the Planet-A probe that would make the close pass of the comet and the MS-T5 (Mu Satellite - Test 5) technology demonstrator, launched seven months earlier in order to test the launch vehicle and the probe design as well as allow distant observations of the interplanetary environment upstream of the comet. The launch vehicle was an upgraded Mu-3SII rocket that used a pair of strap-on boosters to increase the payload to low Earth orbit to 770 kilograms (1,700 pounds) or send up to 150 kilograms (330 pounds) on a direct ascent escape trajectory.

The two spin-stabilized spacecraft, built by Nippon Electronics Corporation, were nearly identical and capable of carrying 12 kilograms (26 pounds) of instruments. MS-T5 had a launch mass of 138.1 kilograms (304.5 pounds) and carried three instruments to characterize the solar wind. It was intended that MS-T5 would pass 5 million kilometers (3 million miles) upstream of Comet Halley near the time of its sister probe’s encounter. Planet A had a launch mass of 139.5 kilograms (307.6 pounds) and carried an electrostatic analyzer to study charged particles and a UV imager based on an instrument flown earlier on the Kyokko (or Exos-A) mission launched in 1978. Because Planet-A carried no dust shield to save mass, it was planned to fly 200,000 kilometers (124,000 miles) from the nucleus where less dust was expected. Because of various launch constraints including those imposed by the powerful Japanese fishing lobby, Planet-A would be launched in August 1985 to make its closet approach near Halley’s descending node, like the ESA and Soviet spacecraft.